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1
Cube the Fontina DOP and place it in a bowl. Pour the whole milk over the cheese and leave to soak for at least 2 hours at room temperature. The milk penetrates the semi-soft paste, pre-hydrating the proteins. The cheese is ready when the cubes have visibly softened and the milk has taken on a faint ivory tint.
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2
Toast the rye bread slices dry in the oven — no butter — until each slice is firm and deeply coloured at the edges. The bread is done when it is rigid enough to resist the fonduta without softening immediately, and the cut surface is dry and slightly rough to the touch.
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3
Set up a bagnomaria: a heatproof bowl set over a saucepan of water maintained below 80 °C — the water must not boil beneath the bowl at any point during cooking. Place the soaked Fontina cubes, the soaking milk, and the butter into the bowl. Stir with patience as the cheese begins to melt into the liquid. This is where the dish lives or dies: if the water boils, the proteins seize and the fat separates into an irretrievable, greasy mass; keep the heat low and maintain a steady, gentle warmth until the mixture is entirely smooth, fluid, and coats the back of the spoon without any lumps.
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4
Remove the bowl from the heat. Add the egg yolks one at a time, whisking constantly after each addition before introducing the next. The fonduta will thicken perceptibly with each yolk and take on a deeper golden colour. The finished emulsion is glossy, uniform, and coats a lifted whisk in a slow, unbroken ribbon. If using white pepper and nutmeg, add them now.
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5
Arrange the toasted rye bread in pre-warmed bowls. Pour the fonduta over the bread immediately — it firms within minutes of leaving the heat. Where white truffle is in season, shave it directly over the poured fonduta at the table. Serve at once.